Process for monitoring bar cutoff on automatic bar lathes

ABSTRACT

The invention pertains to a process for monitoring bar cutoff on CNC-controlled automatic bar lathes having a main spindle (3), an auxiliary spindle (6) and a pull-away carriage (WS3). In cutting-off operations it is necessary to monitor whether the cutting-off operation has cleanly severed the workpiece from the unmachined part. This control prevents machine damage that could arise as a result of imperfectly severed workpieces. The invention provides that after the cutting-off operation is completed, the pull-away carriage is started in the direction of a predetermined control path. In the time the pull-away carriage moves along the control path, the contour variation is monitored and compared with a value for contour variation that was defined in no-load operation, i.e. without a workpiece. If upon comparison the contour variation exceeds a predetermine threshold of difference, i.e. if the contour variation that was defined in no-load operation deviates by a certain value from the contour variation determined over the control path, this indicates that the workpiece was imperfectly severed from the unmachined part and an error signal is generated.

BACKGROUND OF THE INVENTION

The invention concerns a method for monitoring bar cutoff inCNC-controlled automatic bar lathes having a main spindle, an auxiliaryspindle, and a pull-away carriage.

JP-A 3035902 describes a method with which the cutoff tool or thespindle of the tool machine is protected against damage by removing thecutoff workpiece from the spindle along a predetermined path aftercompletion of the cutting-off procedure.

When carrying-out bar cutoff processes it is necessary to check whetheror not the workpiece has been fully severed from the unmachined part.This checking procedure prevents machine damage which could be caused byincompletely severed workpieces.

DE A 36 40 624 describes a diagnostic device with which changes invariable operation parameters at certain typical positions of themachine as well as the reaction times necessary for the changes arestored in a memory during the normal machining (error-free operation) ofa workpiece. During the processing of workpieces the changes in variableoperation parameters as well as the reaction times are determined ineach case at these typical operating positions and compared with valueswhich had been stored during normal processing. In the event that thevalues do not differ by more than a predetermined amount, the processingis continued.

A process for checking bar cutoff on automatic bar lathes is known inthe art through German laid-open publication DE 36 18 349 A1 with whichoperating conditions are initiated at the end of a predetermined timeinterval allotted to the cutting-off procedure, which effect a relativerotation of the spindles with respect to each other when the materialhas been fully severed at the cutoff location. A deviation between thepredetermined relative rotation of the main spindle with respect to theauxiliary spindle and the actual relative rotation indicates theexistence of a material connection, and a corresponding error signal isissued.

This method has the disadvantage that a separate examination procedureis necessary to monitor the cutting-off procedure, which delays the workprocess.

It is therefore the purpose of the invention to introduce a method formonitoring bar cutoff which does not introduce a delay in the workprocess to thereby minimize additional controlling time resulting frommonitoring of the cutoff.

SUMMARY OF THE INVENTION

This purpose is achieved in that the pull-away carriage is started alonga predetermined control path after termination of the cutting-offprocedure; the carriage error is monitored during motion of thepull-away carriage along the control path; the carriage error value iscompared to the value of the carriage error determined for motion of thepull-away carriage along the control path under no-load conditions; andan error signal is issued when a predetermined difference thresholdbetween the two carriage error values is exceeded.

In accordance with the invention, the pull-away carriage is startedalong a predetermined control path after completion of the cutting-offprocedure. During the time the pull-away carriage moves along thecontrol path, the carriage error, e.g. the difference between theintended and the actual location of the pull-away carriage, is monitoredand compared to a carriage error value determined under no-loadconditions, e.g. without a workpiece. If, as a result of the carriageerror comparison, a predetermined difference threshold is exceeded, e.g.a deviation by a predetermined amount between the carriage errordetermined under no-load conditions and the carriage error extractedalong the control path indicates incomplete severing of the workpiecefrom the unmachined part and an error signal is issued.

The invention has the advantage that a separate processing step is notnecessary for monitoring of the cutting-off procedure since, after thecutting-off procedure is ended, it is in any event necessary for theauxiliary spindle to be retracted together with the workpiece in orderto remove the workpiece from the auxiliary spindle. This retraction pathsimultaneously serves as the control path which is then monitored by themethod in accordance with the invention.

An advantageous embodiment of the invention provides that the carriageerror be determined under no-load conditions and after termination ofthe cutting-off procedure during the acceleration phase of the pull-awaycarriage. In this fashion it is possible to immediately recognizewhether the cutting-off procedure was successful or if a materialconnection remains.

Another advantageous embodiment of the invention provides that thecarriage error be determined during the static phase, e.g. aftertermination of the pull-away carriage acceleration phase. In thisfashion more close limits in determining the threshold value can be set,since irregularities in the starting-up phase do not effect measurementsmade during the static phase.

A further advantageous embodiment of the invention does not determinethe carriage error, rather the carriage error slope. In this case, themaximum carriage error slope under no-load conditions is compared to themaximum carriage error slope at the beginning of the control path. Inthe event that a predetermined threshold value for the differencebetween the maximum carriage error slopes is exceeded, the existence ofa material connection is recognized and an error signal issued.

The determination of the carriage error slope has the advantage of beingable to already determine the smallest of material connections duringthe starting-up phase.

In an additional advantageous embodiment of the invention, a momentreduction is activated at the beginning of the control path aftertermination of the acceleration phase, e.g. the drive force acting onthe pull-away carriage is reduced. In the event that the carriage errorslope changes following activation of the moment reduction, an errorsignal is issued. This procedure renders the reference measurement underno-load conditions unnecessary.

A further embodiment of the invention provides for travelling along thecontrol path towards the unmachined part, determining the carriage errorslope or the carriage error, and comparing to the no-load values.

Pieces of material remaining on the cut-off workpiece can be recognizedin the event that a change in the carriage error or in the carriageerror slope occurs within a path length corresponding to the cuttingwidth, i.e. the width of the cutting tool. In this fashion remainingpieces of material which do not form material connections can berecognized.

BRIEF DESCRIPTION OF THE DRAWING

In the following, the invention is described with reference to FIGS. 1and 2.

FIG. 1 shows a cutting-off apparatus and

FIG. 2 shows a diagram of the carriage error under no-load conditionsand along the control path.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a schematic cutting-off apparatus having cutting tools hand2 introduced on carriages WS1 and WS2. The cutting off apparatus alsoincludes a main spindle 3 at which the unmachined part 4 is clamped. Theworkpiece 5, which is to be severed from the unmachined part 4 isattached to the auxiliary spindle 6. The pull-away carriage WS3 isarranged at auxiliary spindle 6. During the cutting-off procedure, theworkpiece 5 is severed from the unmachined part 4, by means ofcutting-off tools 1 and 2. After the cutting-off procedure is completed,the pull-away carriage WS3 with the auxiliary spindle 6 and with thecut-off workpiece 5 is moved in the direction of the Z-axis. This pathsimultaneously serves the purpose of a control path for determining thecarriage error or the slope in the carriage error. The obtained carriageerror or slope in carriage error is compared with a value of thecarriage error or the carriage error slope respectively which had beenpreviously determined under no-load conditions.

Possible functional dependences of carriage error under no-loadconditions and when travelling along the control path are shown in FIG.2. In the figure, the carriage error (in μm) is given as a function oftime t (in ms). The dashed-line shows the carriage error under no-loadconditions and the solid line shows the carriage error when travellingalong the control path. The maximum carriage errors are indicated withS1 (for no-load) and S2 (along the control path). The maximum carriageerror slopes are, for example, indicated with dS1 and dS2 for theno-load condition and long the control path.

FIG. 2 shows a situation with which the workpiece has not been fullysevered, e.g. where a material connection is present so that thecarriage error S2 is larger than the carriage error S1 or the maximumcarriage error slope dS2 is larger than the maximum carriage error slopedS1. A comparison between carriage errors leads, in each case, todetermination of a threshold value S for the carriage error differenceunder no-load and along the control path. The threshold value isdetermined in such a fashion that possible variations in the carriageerror dependence which do not necessarily indicate a materialconnection, do not lead to an error message. The following conditionthereby results

    S<S2-S1,

which must be fulfilled for an error signal to be issued. S is thepredetermined threshold value, S1 the maximum carriage error underno-load conditions and S2 the maximum carriage error along the controlpath.

When monitoring the carriage error slope the condition:

    dS<dS2-dS1

is required for the issuance of an error signal, whereby dS is thedetermined threshold value, dS1 the maximum carriage error slope underno-load and dS2 the maximum carriage error slope along the control path.

In order to also be able to identify material remnants on the workpiecewhich do not lead to material connections, an additional control path inthe -Z-direction can be taken and the carriage error or the carriageerror slope can be determined under no-load conditions. In this casematerial remnants are recognized for travel starting from the initialposition along a path corresponding to the width WB of the cutting-offtool. A difference between the carriage error or the carriage errorslope in comparison to no-load conditions may only occur after this pathlength has been travelled through. A deviation in the carriage error orthe carriage error slope at an earlier location indicates materialremnants on the workpiece or on the unmachined part, whereby a limitingvalue, which must be exceeded, is also set in this case.

I claim:
 1. A method for monitoring bar cutoff in CNC-controlledautomatic bar lathes, the bar lathe having a main spindle, an auxiliaryspindle and a pull-away carriage for supporting a cutoff workpiece, themethod comprising the steps of:moving the pull-away carriage away fromthe main spindle along a control path without the cutoff workpiece;monitoring an unloaded carriage error without the workpiece duringmotion along said control path; moving the pull-away carriage away fromthe main spindle along said control path, with the cutoff workpiece;monitoring a loaded carriage error with the workpiece during motionalong said control path; comparing said loaded carriage error to saidunloaded carriage error; and issuing an error signal if a differencebetween said loaded and unloaded carriage error exceeds a differencethreshold.
 2. The method of claim 1, wherein said unloaded and saidloaded carriage error are each monitored during an acceleration phase ofthe pull-away carriage.
 3. The method of claim 1, wherein said loadedand said unloaded carriage error are each monitored during a staticmotion phase of the pull-away carriage.
 4. A method for monitoring barcutoff in CNC-controlled automatic bar lathes, the bar lathe having amain spindle, an auxiliary spindle and a pull-away carriage forsupporting a cutoff workpiece, the method comprising the steps of:movingthe pull-away carriage away from the main spindle along a control pathwithout the cutoff workpiece; monitoring an unloaded carriage errorslope without the workpiece during motion along said control path;moving the pull-away carriage away from the main spindle along saidcontrol path, with the cutoff workpiece; monitoring a loaded carriageerror slope with the workpiece during motion along said control path;comparing said loaded carriage error slope to said unloaded carriageerror slope; and issuing an error signal if a difference between saidloaded and unloaded carriage error slopes exceeds a slope differencethreshold.
 5. The method of claim 4, further comprising the step ofactivating a moment reduction at a beginning of said control path aftertermination of an acceleration phase, wherein said monitoring of saidunloaded and said loaded carriage error slopes is done after saidactivating of said moment reduction.
 6. A method for monitoring barcutoff in CNC-controlled automatic bar lathes, the bar lathe having amain spindle, an auxiliary spindle and a pull-away carriage forsupporting a cutoff workpiece, the method comprising the steps of:movingthe pull-away carriage towards an unmachined part along a control pathwithout the cutoff workpiece; monitoring an unloaded carriage errorwithout the workpiece during motion along said control path; moving thepull-away carriage towards said unmachined part along said control path,with the cutoff workpiece; monitoring a loaded carriage error with theworkpiece during motion along said control path; comparing said loadedcarriage error to said unloaded carriage error; and issuing an errorsignal if a difference between said loaded and unloaded carriage errorexceeds a difference threshold.
 7. The method of claim 6, wherein saidmonitoring of said loaded and said unloaded carriage error is carriedout along a control path length corresponding to a width of acutting-off tool.
 8. A method for monitoring bar cutoff inCNC-controlled automatic bar lathes, the bar lathe having a mainspindle, an auxiliary spindle and a pull-away carriage for supporting acutoff workpiece, the method comprising the steps of:moving thepull-away carriage towards an unmachined part along a control pathwithout the cutoff workpiece; monitoring an unloaded carriage errorslope without the workpiece during-motion along said control path;moving the pull-away carriage towards said unmachined part from the mainspindle along said control path, with the cutoff workpiece; monitoring aloaded carriage error slope with the workpiece during motion along saidcontrol path; comparing said loaded carriage error slope to saidunloaded carriage error slope; and issuing an error signal if adifference between said loaded and unloaded carriage error slopesexceeds a slope difference threshold.
 9. The method of claim 8, whereinsaid monitoring of said loaded and said unloaded carriage error slope iscarried out along a control path length corresponding to a width of acutting-off tool.